A traditional solid oxide fuel cell comprises a cathode, an anode and an electrolyte. Often, perovskites of the general form ABO3-δ are used as the cathode active material. In such a configuration, A and B both represent cations, and these cations have historically both had charges of approximately +3.
The primary function of the cathode in the solid oxide fuel cell is to facilitate the electrochemical reduction of oxygen, which requires the diffusion of oxygen through the cathode. To that end, the use of +3 charged cations in the perovskite cathode material has long been thought to impart the fastest rate of oxygen diffusion. However, even with this configuration, oxygen diffusion remains the rate limiting step in the electroreduction process.
Notwithstanding the rate limiting nature of oxygen diffusion in cathodes utilizing this perovskite configuration, solid oxide fuel cells employing these cathode materials have exhibited satisfactory power generation at very high temperatures, i.e. 800-1000° C. Nonetheless, such high operating temperatures lead to high costs and limit material compatibility. For example, conventional solid oxide fuel cells use yttria-stabilized zirconia (YSZ) as an electrolyte. In these fuel cells, the transition metal perovskite (La1-xSrx)MnO3-δ (LSM) has traditionally served as the cathode. However, the electrochemical reduction of oxygen over LSM creates a high activation energy, rendering the LSM cathode material inappropriate for reduced temperature operation.
Efforts have been made to develop a cathode material suitable for reduced temperature operation. However, these efforts have focused on mixed electron and oxygen ion conducting perovskites such as doped LaCoO3, doped LaFeO3 and doped SmCoO3. For example, the perovskites La1-xSrxCoyFe1-yO3-δ (LSCF) and Sm0.5SrO0.5CoO3-δ (SSC) have shown particularly high activities in the 600 to 800° C. temperature range. Although these cathode materials exhibit substantially improved performance compared to LSM, no cathodes suitable for operation at temperatures less than 600° C. have yet been developed. Furthermore, these perovskite cathode materials are far too active for propane catalytic oxidation in high efficiency single chamber fuel cells. Accordingly, a need arises for a perovskite cathode material that exhibits accelerated oxygen diffusion, and that is suitable for reduced temperature operation in both single and dual chamber fuel cells.